This invention relates to post-weld thermal treatments and methods. More particularly, this invention is directed to methods for improving the mechanical properties of seam welds with reduced weld-zone hardness and improved weld-zone ductility and toughness on hardenable ferrous alloys.
Ubiquitously used throughout all modern industries, welded ferrous alloys have become the de facto standard in structural component design. Current trends in many fields have focused interest away from low-strength common mild steels towards high and ultra high strength steels. These alloys are formulated to have greater tensile strengths than low-carbon steels, due to the specific microstructures that are produced during thermomechanical processing. Some examples of high strength steels currently in use in the automotive industry include dual-phase, martensitic, boron-treated, and transformation-induced plasticity steels. Other high-strength alloys include air, oil and water hardenable carbon steels and martensitic stainless steels. All of these are designed so that some volume percentage of martensite forms in the material's microstructure. The resulting distorted body-centered cubic (BCC) or body-centered tetragonal (BCT) martensitic crystal structure formed in the hardened condition imparts high strength to the metal. These materials are ideally suited for structural components and assemblies, satisfying the requirements of high strength and toughness.
Unfortunately, the tendency to form martensite, and relative high hardenablity, of these and other ultra high-strength alloys poses difficulties in welding. The thermal cycle of heating and rapid cooling, which occurs within the confined heat-affected-zone (HAZ) during welding, is equivalent to a rapid quenching cycle. The chemistry of high strength steel grades results in a complete transformation from ferrite to austenite (γ) at high temperature, followed by a subsequent change to the hard martensite phase upon rapid cooling. In seam welding applications, the natural weld cooling rate can be as high as 1000° C./s, sufficiently fast enough to produce a martensitic structure in most high strength, high-carbon alloys. (See FIGS. 1 and 2). The resulting martensitic structure produced is extremely brittle in the untempered condition. Cracking of the weld zone can occur for several reasons, including:                Hydrogen induced cold-cracking, due to trapped hydrogen in the distorted BCC martensite crystal structure. Tensile stress applied to the weld increases the risk of cracking.        Thermal induced stresses, due to the heat input during welding, degree of joint restraint, and the volume change upon martensite transformation.        
Most forms of cracking result from shrinkage strains that occur as the weld metal cools to ambient temperature. If the contraction is restricted, the strains will induce residual tensile stresses that cause cracking. There are two opposing forces: the stresses induced by the shrinkage of the metal, and the surrounding rigidity of the base material. Large weld sizes, high heat input and deep penetrating welding procedures increase the shrinkage strains. The stresses induced by these strains will increase when higher strength filler metals and base materials are involved. With higher yield strengths, higher residual stresses will be present.
These problems occur when welding certain steels regardless of their prior condition, whether annealed, hardened, or hardened-and-tempered. They can occur with all types of welding, including GTAW, GMAW, SAW, PAW, laser-beam, friction, resistance and electron-beam. In all cases, the fusion zone and high-temperature HAZ will be in the “as-quenched” condition after welding. Any mechanical straining after welding, either in secondary fabrication processes or in service, may cause the martensitic HAZ to crack.
Additionally, many assemblies, once welded and fabricated from these alloys, cannot be subjected to a final homogenizing solution heat treatment cycle. Examples include assemblies fabricated from pre-hardened or special thermomechanically processed base metals, such as dual phase steels, whereby the heat cycle would destroy the unique microstructure of the alloy. Also, placing the entire welded assembly into a furnace to be post-weld stress relieved may not be physically feasible, as the case of automotive structural beams welded to the massive vehicle body structure. Some assemblies would not tolerate an entire-structure post-weld thermal treatment, as is the case for welded automotive fuel tank assemblies with thermoplastic interior components. In any case, great benefits could be realized if the as-welded brittleness could be reduced. Ductility and toughness of the finished weld would be greatly improved in the case of welded structures put into service without any further thermal treatment.
Typical methods of controlling weld and HAZ hardness include off-line secondary post-weld heat treatments (PWHT) such as process annealing and tempering of the weld by heating the entire part. Pre-heating methods can be used to slow the rate of cooling, thereby reducing the percentage of the martensitic phase present. The latent heat in the workpiece reduces the cooling rate of the welded seam, and cracking is thus inhibited. In the past, pre- and post-weld heat treatments have been performed in large batch heat treatment furnaces to ramp and hold a group of components at a suitable heat treatment temperature. Drawbacks to the use of batch heat treatment processes include long heat treatment times, due in part to the mass of the large batch furnace and the mass of the components being heat treated. Additionally, long queuing times occur while batches are assembled as individual components are welded. Standard post-weld thermal treatments, such as stress relieving or tempering, involve relatively long hold times at prescribed temperatures, on the order of several hours, along with slow furnace cooling. To compound matters more, a global pre-or-post-weld heat treatment can destroy the desired microstructure of the base metal. Parts made from dual phase or martensitic steels, for example, may suffer an overall loss in mechanical properties if the entire part is subjected to a thermal treatment with other-than optimal heating times and quench rates.
It is well known in fabrication shops to re-heat welds to re-austenitize the seam weld with an oxyacetylene torch and allow the joint to cool slowly in still air when working with hardenable steels, such as chromoly alloys. The heat retained by the weld joint and surrounding metal effectively slows the cooling rate of the weld after re-heating, reducing the brittleness, leaving the joint in a softer, normalized condition. However the process is variable, slow, piece-wise and the results are highly dependent on the fabricator's skill level. Alternative methods, such as those described by U.S. Pat. No. 3,046,167, entitled Heat-Treating Method and Product, provide for the re-austenitization of the weld seam with torches followed by slow cooling. Similar methods are described in U.S. Pat. No. 6,676,777, entitled Postweld Heat Treatment Process of Carbon Steel and Low Alloy Steel, which prescribes reheating the weld to the austenite region and holding for some time, followed by “slower than air” cooling. All of these methods rely on reheating the weld seam above the upper-critical temperature and slow cooling to produce the desired microstructure.
Conventional controlled “localized” post-weld stress relieving is often applied to severe service industrial and oilfield piping welded joints. Heat is applied through heating “blankets” which consist of inductive or resistance heating coils wrapped around the joint's circumference. The heat is applied very slowly, allowed to dwell at a peak temperature for several minutes to several hours, and then allowed to cool very slowly in the insulative heating blanket.
In seam-welded tube production, the traditional approach to solve welding difficulties inherent of high-strength alloys is to modify the material's chemical composition. Typically, low-carbon versions of air-hardenable alloys are developed so that the seam-weld does not become fully martensitic and will not crack during tube production. An example of this is U.S. Pat. No. 7,157,672 entitled Method of Manufacturing Stainless Steel Pipe For Use In Piping Systems, which details the use of low-carbon dual-phase 0.08% C max stainless material in conventional tube manufacturing processes. Similarly, a modified composition is used to produce pipe in the following article: Development of weldable martensitic stainless steel line pipe by HF-ERW process, N. Ayukawa, et al., Stainless Steel World 1999 Conference Proceedings, 1999. In modifying the chemical composition, there is a tradeoff of between the ease of welding and the material's hardenability and maximum mechanical properties.
Another method to reduce weld hardness is to add filler material, whereby the final metallurgy is modified in such a way that the percentage of hard and brittle components such as martensite is reduced. However, some seam welding processes (such as laser or resistance) are difficult to use with filler metals. Additionally, costly filler metals are selected so as to not harden upon cooling, and thereby provide lower strength weldments. This necessitates an even larger weld to be used to meet the required joint strength.
Other methods to improve seam weld properties include mechanically straining and working the weld to induce residual compressive stresses and thereby reduce the tendency of weld seam cracking. This method is not effective, or even possible, for all but the simplest seam weld geometries. U.S. Pat. No. 4,072,035 entitled Strengthening of a Welding Seam, details this method.
It is well known in seam-welded tubing production to use “seam annealers” to improve the mechanical properties of the seam weld. These devices, designed to operate on non-hardenable alloys such as low-carbon steels and austenitic stainless steels, apply a secondary heat source on the seam weld downstream of the weld source after it cools fully to ambient temperature. Two main distinctions apply: firstly, “seam annealers” reheat the weld to above the AC3 temperature, re-austenitizing the material and hold for some time then allowing to slow cool, equivalent to a “normalizing” heat treatment cycle; secondly, “seam annealers” are employed on non-hardenable alloys. Examples of the “seam-annealing” processes are described in U.S. Pat. No. 3,242,299 entitled Inductor for Induction Heating Apparatus, and U.S. Pat. No. 4,975,128, entitled Method For Heat-Treating Straight Bead Welding Pipes For Use in Piping Systems.
U.S. Pat. No. 2,293,481 entitled Welding Apparatus and U.S. Pat. No. 2,262,705 entitled Electric Welding describe methods to produce welds with reduced brittleness. Both methods utilize a relatively short tempering cycle on hardenable alloys, reheating the weld to improve mechanical properties. However, these methods employed on band-saw blades and spot welds differ from the present invention. These processes are performed in-situ using the same equipment as used to produce the weld. Indeed, the spot welding equipment must stay in place for proper quenching for the method of U.S. Pat. No. 2,262,705, then followed immediately by re-heating to temper the weld. Most significant is that these processes are employed on discrete weld joints: i.e., spot, flash, or projection type welds.
Conventional processes such as batch pre-heating and PWHT do not lend themselves to cost-efficient, high-quality, high volume production. Unfortunately, these methods are not cost, time, nor energy efficient for high production levels associated with modern manufacturing methods. The ideal solution would allow for either autogenous welds (i.e. no filler metal used) or the use of matching strength filler metals, of similar chemical compositions to the base metals being welded that are capable of hardening to a high strength joint, combined with rapid heating and inexpensive air-cooling cycles.
The present inventors describe various methods for increasing weld and HAZ ductility within U.S. Pat. No. 7,232,053 issued Jun. 19, 2007; U.S. Provisional Application Ser. No. 60/879,861, filed Jan. 10, 2007; U.S. application Ser. No. 11/542,970, filed Oct. 4, 2006; U.S. application Ser. No. 11/526,258 filed Sep. 22, 2006; U.S. application Ser. No. 11/519,331, filed Sep. 11, 2006;U.S. application Ser. No. 10/519,910, filed Dec. 30, 2004; International Application Ser. No. PCT/US02/20888 filed Jul. 1, 2002; U.S. Provisional Application Ser. No. 60/301,970, filed Jun. 29, 2001. Each of these references are incorporated by reference in their entirety herein. Unfortunately, even these methods have disadvantages.
Thus, it would be desirable from a production point of view to provide a heat treatment during production would be preferred in order to improve the mechanical properties of seam welded joints. Preferably, a simple PWHT method could be used to appreciably increase weld and HAZ ductility.